perf(images): convert remaining <img> tags to next/image project-wide

Clears every remaining @next/next/no-img-element warning — automatic
responsive srcset, lazy-loading, and format optimization instead of
always loading the original file at full size. Fixed-size icons got
explicit width/height; dynamic-aspect photos got fill inside a
relative wrapper.

app/components/Hero.tsx

@@ -61,11 +61,7 @@ export function Hero() {
               Starte mit der 7-Tage-Challenge
             </span>
             <div className="relative h-[1.1875rem] w-[1.5625rem] shrink-0">
-              <img
-                alt=""
-                src="/icon-check.svg"
-                className="absolute inset-0 w-full h-full"
-              />
+              <Image alt="" src="/icon-check.svg" fill sizes="26px" />
             </div>
           </Link>